1. Field of the Invention
The present invention relates to an endoscope system having a zoom function.
2. Description of the Related Art
Conventionally, endoscope systems have been widely used in a medical field and an industrial field. An image of an object is picked up by an image pickup device provided in a distal end portion of an insertion portion of an endoscope, and the object image is displayed on a monitor apparatus. A surgeon or the like can make an observation by viewing the image of the object appearing on the monitor. An image pickup device and an observation optical system are incorporated in a distal end portion of an insertion portion of an endoscope.
In recent years, as disclosed in Japanese Patent Application Laid-Open Publication No. 2009-148369, endoscope apparatuses including a mechanism that moves a lens barrel of an observation optical system in an optical axis direction for a two-focus focusing function for an object image, in an insertion portion have been proposed. For an actuator that moves the lens barrel, a shape memory alloy wire is used, and the wire is expanded/contracted by controlling current flowing in the wire, whereby a focusing function of an observation optical system is provided.
Also, for example, Japanese Patent Application Laid-Open Publication No. 2010-48120 proposes an actuator system that memorizes a maximum resistance value and a minimum resistance value when current is made to flow in a shape memory alloy wire, to perform resistance control based on a value of current flowing in the wire.
Note that a technique that improves, e.g., responsiveness for lens movement in two-focus optical zoom operation is proposed in Japanese Patent Application No. 2010-156156 (not published yet) filed by the present applicant.